The Jumonji C domain-containing histone demethylases (JmjC-HDMs) are α-ketoglutarate (αKG)-dependent, O(2)-activating, non-heme iron enzymes that play an important role in epigenetics. Reported herein is a detailed kinetic analysis of three JmjC-HDMs, including the cancer-relevant JMJD2C, that was achieved by employing three enzyme activity assays. A continuous O(2) consumption assay reveals that HDMs have low affinities for O(2), suggesting that these enzymes can act as oxygen sensors in vivo. An interesting case of αKG substrate inhibition was found, and the kinetic data suggest that αKG inhibits JMJD2C competitively with respect to O(2). JMJD2C displays an optimal activity in vitro at αKG concentrations similar to those found in cancer cells, with implications for the regulation of histone demethylation activity in cancer versus normal cells.